Insights into the chemistry and structure of iron gall ink based on “two-step” bamboo nib-spray/mass spectrometry

A study of the dark-blue insoluble precipitate that makes up gall ink using bamboo nib-spray mass spectrometry (PS-MS) is described. A ferrous tannate complex is formed, i.e. an insoluble organic metal complex - iron gallate ink (IGI) is produced when tannin is mixed with iron sulfate. Since it is not possible to detect this IGI precipitate by conventional electrospray ionization (ESI) mass spectrometry, we report on an attempt to resolve this problem by a new method in this study in an. The method involved “two steps” using bamboo nib-spray/MS. A gallic acid solution was initially used as a model sample and the ESI mode was operated under a negative voltage. This approach permitted the optimized position between the spray-tip and the mass inlet (either distance or angle) to be determined. Following this, the ESI applied voltage was changed to positive and, at this moment, a ferrous sulfate solution was added to the spray-tip, both reactants are sprayed out immediately and are simultaneously ionized while a complex binding reaction occurs. This permits the IGI dark-blue insoluble precipitate to be ionized and detected. As a result, several specific peaks could be observed. The resulting m/z = 153, 225, 243, 431 peaks are assigned to [C₇H₅O₄]⁺, [C₇H₅O₅Fe]⁺, [C₇H₅O₅Fe‧H₂O]⁺, and [(C₆H₃O₃) (C₆H₄O₃)Fe₂‧4H₂O]⁺, respectively.